Can anyone solve this

The product of two negative rational numbers is always zero sometimes zero never zero

For the rule of 78, for a 12-month period, the last term in the sequence is 12 and the series sums to 78. for an 10 month period, the last term is and the series sum is . for a 15 month period, the last term is and the series sum is . for a 20 month period, the last term is and the series sum is

Asphere has a diameter of 10 in. what is the volume of the sphere?